Super-Sharp Images Through Thin Optical Fibers

An advanced wavefront shaping method combined with unique optical fibers make it possible to focus lensless light at an unparalleled resolution.

Super-sharp images from within the human body made through tiny endoscopes have come a step closer to reality thanks to joint research by scientists from the University of Twente's (UT) MESA+ research institute, the Max Planck Institute for the Science of Light (MPL), Foundation for Fundamental Research on Matter (FOM), and Carl Zeiss AG. An advanced wavefront shaping method developed at the UT combined with unique optical fibers from MPL make it possible to focus lensless light at an unparalleled resolution. FOM postdoc Dr. Lyubov Amitonova and her colleagues published their findings recently.

Optical imaging via ultrathin fiber probes is extremely useful for taking a look inside the human body in a minimally invasive manner. Unfortunately, the resolution of current fiber endoscopes is one micrometer at best. This is not enough to see interesting and important fine features in biological cells, for example. Some endoscopes use a high number of separate fibers bound together into a fiber bundle. Each fiber then acts like a discrete pixel to form the final pixelated image. However such bundles tend to be quite thick, at least a millimeter in diameter.

An alternative is fiber endoscopes based on 'multimode' fibers. These could offer imaging with a better view and be as thin as a tenth of a millimeter. A multimode fiber uses only a single fiber core that can transmit an entire image. Unfortunately, the image becomes scrambled as it passes through the fiber. However, some tricks for unscrambling these images are available. The main limiting factor for the resolution of these multimode endoscopes is that the fibers only transmit light that propagates along the fiber axis. Light under a small angle can still bounce from the fiber walls. But if this angle gets too large, the light will simply leak out to the side. FOM postdoc Dr. Lyubov Amitonova and her colleagues at UT and MPL have now shown that with photonic crystal fibers this limitation can be overcome.

Unique crystal fiber probe

Conventional ('step-index') fibers consist of two zones of different material (an outer cladding and an inner core) with distinct refraction indices, which enable light transmission down the fiber axis by total internal reflection. Photonic crystal fibers are built differently: they are made of one material only and light guiding is realized through the presence of a specific pattern of holes in the cladding, which are filled with air. Tailoring the cladding structure of such a fiber provides a unique tool for engineering specific fiber-optic properties. In this project, the scientists have designed and made such a fiber to focus a laser beam through the fiber down to 0.52 micrometers, using visible red light.

Sharp focus and high resolution

A photonic crystal fiber acts as a multimode fiber in which the image typically gets scrambled due to light bouncing off the possibly irregular wall of the fiber. The technique of complex wavefront shaping, invented at UT, is able to undo such scrambling and make a sharp focus. This is achieved by pre-shaping the light into the precise form needed to make a sharp image behind the fiber before the light actually enters the fiber. Using this approach, Amitonova and her colleagues have succeeded in focusing light at the fiber output facet of different multimode fibers including several unique photonic crystal fibers. They have shown that the complex wavefront shaping technique together with a properly designed multimode photonic crystal fiber allows the creation of a tightly focused spot at the desired position on the fiber output facet with a subwavelength beam waist. This paves the way towards high-resolution endoscopic imaging via fiber probes so thin that they could be inserted, for instance, into tiny blood vessels not much thicker than a human hair.

Illustration: Electron microscope image of one of the photonic crystal fibers made at MPL for this research. The glass is black, the air is grey, and the red dot illustrates the focus.
Credit: Image courtesy of University of Twente.